Optical temperature sensors are known that use a temperature sensing element made of a semiconductor having an energy gap that varies in response to temperature changes (see, e.g., Patent Documents 1-5). Such optical temperature sensors are configured to transmit signal light emitted from a first light emitting element and reference light emitted from a second light emitting element through the temperature sensing element, and detect an external temperature based on the light intensities of the signal light and the reference light that have been transmitted through the temperature sensing element.